HCCI (homogeneous charge compression ignition) is a recognized process for fueling a diesel engine in a manner that creates a substantially homogeneous air-fuel charge inside an engine cylinder during a compression upstroke of an engine cycle. After a desired quantity of fuel for the charge has been injected into the cylinder to create a generally homogeneous air-fuel mixture, the increasing compression of the charge by the upstroking piston creates sufficiently large temperature to cause auto-ignition of the charge near or at top dead center (TDC). Auto-ignition may occur as the substantially simultaneous spontaneous combustion of vaporized fuel at various locations within the mixture.
One of the attributes of HCCI is that relatively lean, or dilute, mixtures can be combusted, keeping the combustion temperatures relatively low. By avoiding the creation of relatively higher combustion temperatures, HCCI can yield significant reductions in the generation of NOx, an undesired constituent of engine exhaust gas.
Another attribute of HCCI is that auto-ignition of a substantially homogeneous air-fuel charge generates more complete combustion and consequently relatively less soot in engine exhaust.
The potential benefit of HCCI on reducing tailpipe emissions is therefore rather significant, and consequently HCCI is a subject of active investigation and development by many scientists and engineers in the engine research and design community.
HCCI may be considered one of several alternative combustion processes for a compression ignition engine. Other processes that may be considered alternative combustion processes include Controlled Auto-Ignition (CAI), Dilution Controlled Combustion Systems (DCCS), and Highly Premixed Combustion Systems (HPCS).
By whatever name an alternative combustion system or process may be called, a common attribute is that fuel is injected into a cylinder well before TDC to form an air-fuel charge that is increasingly compressed until auto-ignition occurs near or at top dead center (TDC).
If such alternative processes are not suitable over the full range of engine operation for any particular engine, the engine may be fueled in the traditional conventional diesel manner where charge air is compressed to the point where it causes the immediate ignition of fuel upon fuel being injected into a cylinder, typically very near or at top dead center where compression is a maximum.
With the availability of processor-controlled fuel injection systems capable of controlling fuel injection with precision that allows fuel to be injected at different injection pressures, at different times, and for different durations during an engine cycle over the full range of engine operation, a diesel engine becomes capable of operating by alternative combustion processes and/or traditional diesel combustion. The advent of variable valve actuation systems allows timing of engine valves to be processor-controlled in various ways, and with precision.
As will be explained by later description, the present invention takes advantage of the capabilities of such processor-controlled fuel and valve actuation systems to better control auto-ignition when a compression ignition engine is operating in an alternative diesel combustion mode.
Because a diesel engine that powers a motor vehicle runs at different speeds and loads depending on various inputs to both the vehicle and the engine that influence engine operation, fueling requirements change as speed and load change. An associated processing system processes data indicative of parameters such as engine speed and engine load to develop control data for setting desired engine fueling for particular operating conditions. A control algorithm seeks to secure fuel injection system operation that will provide the desired fueling at each of various combinations of engine speed and engine load.
A variable valve actuation system can also be controlled in different ways according to different engine speed-load conditions to provide effective compression ratio appropriate to each of multiple combinations of those conditions. A control algorithm seeks to secure a desired effective compression ratio that in conjunction with fueling determined by the fuel control algorithm will cause auto-ignition of an in-cylinder mixture to occur at a desired time in the engine cycle that creates desired torque at the particular engine speed.
Even with good control of both fueling and cylinder valve timing, disturbances that create cylinder-to-cylinder variations and/or cycle-to-cycle variations in auto-ignition and resulting torque may be present in an engine. Too early auto-ignition can cause certain undesired effects like potentially damaging engine knock. Too late auto-ignition can result in power loss. These disturbances may be inherent in the design of a particular engine, typically because each cylinder is at a different location in the engine and may therefore operate at a small but nonetheless significant temperature difference from other cylinders and/or may at a different distance from the point at which charge air enters the intake manifold than other cylinders.